Turbine engines are used as the primary power source for various kinds of aircrafts. The engines are also auxiliary power sources that drive air compressors, hydraulic pumps, and industrial gas turbine (IGT) power generation. Further, the power from turbine engines is used for stationary power supplies such as backup electrical generators for hospitals and the like.
Most turbine engines generally follow the same basic power generation procedure. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more air into the engine and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, electrical generators, or other devices.
Since turbine engines provide power for many primary and secondary functions, it is important to optimize both the engine working life and the operating efficiency. However, technology advancements and engine efficiency improvements have led to rising gas temperatures inside an engine during operation. It is important for turbine blades and vanes to maintain their mechanical strength when exposed to heat, oxidation, and corrosive environments associated with an impinging gas. Blades, vanes, and other components are often coated with a thermal barrier coating to insulate the coated components, to inhibit oxidation and gas corrosion, and thereby prolong their workable life.
Some conventional thermal barrier coatings include a lower bond coat layer and an upper ceramic layer. Common formation methods for the upper ceramic layer include plasma spraying and electron beam physical vapor deposition (EB-PVD). Application by EB-PVD is particularly effective as it produces a ceramic layer having a columnar grained microstructure. Gaps between the individual columns allow for columnar grain expansion and contraction without developing stresses that could result in spalling.
One class of columnar thermal barrier coatings that may be produced by EB-PVD has a nanolaminate structure including hundreds to thousands of layers. Each layer includes a ceramic material such as stabilized zirconia and stabilized hafnia. Thermal barrier coatings having a nanolaminate structure provide sufficiently low thermal conductivity to protect turbine engine components such as blades and vanes during engine operation. However, there is a need for additional thermal barrier coatings having lower thermal conductivity. There is a further need for highly stable and durable thermal barrier coatings having compliant microstructures.